This invention relates generally to a power line communication system and more particularly, to a power line communication system for communicating commands and data on a power line.
Electric power supply companies typically have a service person visit customer sites to read an electricity meter at the site to determine the amount of energy consumed for billing purposes. This can be cumbersome and time consuming, particularly when there are many customer sites over a large geographic area.
Many utility meter based systems have been proposed which utilize automated meter reading (AMR) operations. Some systems involve, for example, power line or radio communications. Other systems are hybrids, i.e., involve both power line and radio communications. Typically, data and commands can be transmitted between a central station and the utility meters. Such communication systems enable utilities to easily and quickly obtain energy consumption measurements at many different sites.
Geometric Harmonic Modulation (GHM) signals for transmission on the power line is particularly well suited for AMR applications since GHM signals can be transmitted through power line components, such as distribution transformers. GHM is described, for example, in Hershey et al., U.S. Pat. No. 5,844,949, which is assigned to the present assignee, and is herein incorporated by reference. Generally, GHM allocates signaling energy into lobes, or tones, or different frequencies evenly spaced. The GHM signaling waveforms are true spread spectrum signals in that the signal bandwidth, i.e., the bandwidth from the lowest frequency tone to the highest, exceeds the information bandwidth conveyed by the GHM transmission.
In addition to performing meter reading functions, it is desirable to provide additional functions such as residential utility load shedding. For example, it would be desirable to provide an inexpensive, controllable breaker that utilizes a GHM based extended area AMR system, wherein additional information is communicated over the GHM signal.
These and other objects may be attained by a receiver which can be added to an existing network for enabling one-way, GHM based communications, which do not interfere or reduce the capability of, or limit the normal functions of, the GHM network. More specifically, the receiver detects a small modulation factor signal present on the GHM signal normally transmitted by the GHM transmitter. If the frequency of the modulated signal corresponds to a pre-defined frequency, then the receiver generates a command signal to an external device, such as a relay.
More particularly, in one embodiment, the receiver includes a filter which partially isolates the GHM energy (normally in the 5-10 KHz range) from the lower 60 Hz harmonics. The receiver also includes a envelope detector coupled to the filter. The detector may, for example, be a square wave detector. The envelope detector is coupled to a frequency detector and the frequency detector is coupled to a command decoder. The decoder determines whether at least one amplitude modulated frequency is present on the received signal. If the signal has a known modulation frequency, then the decoder generate a signal which causes a signal generator to generate a command signal for transmission to the external device.